Immune responses to antigen are classified as being predominantly either cell-mediated, exemplified by the phenomena of delayed-type hypersensitivity (DTH), or humoral, exemplified by the production of antibodies. Cell-mediated immunity is of paramount importance for the rejection of tumors and for recovery from many viral, bacterial, protozoan, and fungal infections. In contrast, a humoral immune response is the most effective form of immunity for eliminating toxins and invading organisms from circulation. It has been observed that for different antigens one or the other of these two responses often predominates in a mutually exclusive fashion, and that the severity of some diseases, e.g. leprosy, leishmaniasis, and some types of autoimmunity, may be due to the inappropriate dominance of one class of response over the other, Mosmann et al, Immunol. Today, Vol. 8, pgs. 223-227 (1987); Mosmann et al, Ann Rev. Immunol., Vol. 7, pgs. 145-173 (1989); Parish, Transplant, Rev, Vol. 13, pgs. 35-66 (1972); and Liew, Immunol. Today, Vol. 10, pgs, 40-45 (1989). It has further been observed that sets of cytokines are separately associated with DTH reactions and humoral immune responses, Cher et al, J. Immunol., Vol. 138, pgs. 3688-3694 (1987); and Mosmann et al (1987 and 1989, cited above), and it is thought that diseases associated with these classes of response are caused by the inappropriate production of the associated sets of cytokines.
For example, a large body of evidence suggests that excessive production of gamma interferon (IFN-.gamma.) is responsible for major histocompatibility complex (MHC) associated autoimmune diseases: Hooks et al, New England J. Med., Vol. 301, pgs. 5-8 (1979) (elevated serum levels of IFN-.gamma. correlated with autoimmunity); Basham et al, J. Immunol., Vol. 130, pgs. 1492-1494 (1983) (IFN-.gamma. can increase MHC gene product expression); Battazzo et al, Lancet, pgs. 1115-1119 (Nov. 12, 1983) (aberrant MHC gene product expression correlated with some forms of autoimmunity); Hooks et al, Ann. N.Y. Acad. Sci., Vol., pgs. 21-32 (1980) (higher IFN-.gamma. levels correlated to greater severity of disease in SLE patients, and histamine-release enhancing activity of interferon can be inhibited by anti-interferon sera); and Iwatani et al, J. Clin. Endocrin. and Metabol., Vol. 63, pgs. 695-708 (1986) (anti-IFN-.gamma. monoclonal antibody eliminated the ability of leucoagglutinin-stimulated T cells to induce HLA-DR expression). It is hypothesized that excess IFN-.gamma. causes the inappropriate expression of MHC gene products which, in turn, causes autoimmune reactions against the tissues whose cells are inappropriately expressing the MHC products and displaying autoantigens in the context of the products.
In the area of clinical parasitology, it has recently been observed that the levels of IFN-.gamma. and IL-2 are important factors in the progression and/or resolution of the protozoan infection, leishmaniasis. In particular, the presence of adequate levels of IFN-.gamma. appears to be essential for the activation of infected macrophases to eliminate intracellular amastigotes, Mauel and Behin, in Cohen et al, eds., Immunology of Parasitic Infections (Blackwell, London, 1982). And, in murine models of the disease, it has been shown that high levels of IFN-.gamma. and low levels of IL-4 are associated with resolution, whereas low levels of IFN-.gamma. and high levels of IL-4 are associated with progression of leishmaniasis, Heinzel et al, J. Exp. Med., Vol. 169, pgs. 59-72 (1989).
In view of the above, it would be advantageous to have available agents that could shift the dominance of one class of immune response to the other, and in particular that could suppress or increase the synthesis of IFN-.gamma. and/or other cytokines, respectively, as required for therapy. Such agents would be highly advantageous for treatment of diseases associated with inappropriate or inadequate immune responses, such as tissue rejection, leishmaniasis and other parasitic diseases, and MHC associated immune disorders including rheumatoid arthritis, systemic lupus erythematosus (SLE), myasthenia gravis, insulin-dependent diabetes mellitus, thyroiditis, and the like.